1. Field of the Invention
This invention relates to motor control systems in which a direct current motor is started by the sequential application of direct current until a predetermined speed is reached and maintained at said predetermined speed by application of fixed duration pulses equal in frequency to reference pulses having a proportional relationship to desired rotor velocity.
2. Description of the Prior Art
Motor control systems are known in which a motor is started and accelerated in a first mode of operation until reaching a predetermined angular velocity and then run in a second mode of operation to more adequately assure that the predetermined angular velocity is maintained. Snychronous motor operation is one operating mode often employed to maintain a substantially constant predetermined velocity; however, conventional synchronous motors generally require auxiliary windings or auxiliary prime mover means to accelerate the rotor to the synchronous speed.
The prior art includes systems for operating motors, other than motors primarily designed for operation as synchronous motors, in the mode for which they were designed for starting and acceleration and in a second mode, which may be broadly categorized as synchronous operation, for maintaining a predetermined speed. For example, U.S. Pat. Nos. 3,518,516 and 3,601,678 teach systems in which a stepping motor having a multi-toothed rotor is brought up to a predetermined speed by sequentially driving the stator windings in a step-by-step fashion with oscillator pulses generated by a resettable oscillator in response to emitter pulses indicative of rotor movement. Upon reaching the predetermined speed the windings are driven by oscillator pulses from the resettable oscillator, irrespective of emitter pulses, until a speed change occurs, at which time the system reverts to the first, stepping mode of operation until the predetermined speed is again reached. One problem with this system is that stepping motors are relatively expensive compared with, for example, permanent magnet, direct current motors. Another problem is that the resettable oscillator cannot be easily crystal controlled, although crystal control would be advantageous in systems in which a predetermined speed proportional to the frequency of a train of reference pulses is desired. Further, because of flux variations during rotation by virtue of the multi-toothed rotor of the stepping motor taught by these patents, it would be impractical to attempt to use this motor system for smooth operation at relatively slow speeds.
In another example of the prior art, U.S. Pat. No. 3,783,357 teaches a system in which pulses from a reference oscillator set latches to provide direct current for starting an electronically commutated DC motor. With each rotor revolution the latches become reset in response to rotor position. When the rotor reaches the system design velocity, the frequency of drive pulses to the windings is equal to the frequency of the reference pulses. However, the drive pulses are terminated in response to rotor position, so that the drive pulses have varying width. Terminating the motor drive pulses in response to rotor position assures that the drive pulses enable the production of only positive torque by the rotor which, in instances wherein the mechanical load on the rotor is suddenly decreased, can result in an acceleration of the rotor above the synchronous speed. The solution to this problem taught by the patent takes the form of logic circuitry to automatically change the drive voltage to the windings when minimum and maximum drive pulse widths are exceeded due to changes in torque; however, this solution requires the motor to temporarily drop out of synchronism for each voltage change to take place which, results in imprecise speed regulation, particularly at low speeds.
It would, therefore, be advantageous to provide a system in which an inexpensive motor can be operated with precise speed regulation at predetermined speeds, including substantially low speeds, and over a wide range of torque values while maintaining the precise speed regulation. It would further be advantageous to provide for starting, accelerating, and stopping the motor in such a system in a manner not requiring elaborate logic circuitry or auxiliary windings. It would also be advantageous to achieve very smooth operation of this motor at very low speeds. It would also be advantageous to achieve a closely regulated, hyperbolic speed variation of the motor in accordance with a linear change in the position of the wiper of a linear potentiometer.